1. Field of the Invention
The present invention relates to a pulse modifier, a lithographic apparatus and a method for manufacturing a device
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Lithographic apparatus include large expensive lens elements that are difficult to fabricate. Typically, an excimer laser is used to supply the lithographic apparatus with radiation in the form of pulses. The expensive lens elements are subject to degradation resulting from billions of these high intensity ultraviolet pulses. Optical damage is known to increase with increasing intensity (i.e., light power (energy/time) per cm2 or mJ/ns/cm2) of the pulses from the laser. The typical pulse length from these lasers is about 20 ns, so a 5 mJ laser pulse would have a pulse power intensity of about 0.25 mJ/ns (0.25 MW). Increasing the pulse energy to 10 mJ without changing the pulse duration would result a doubling of the power of the pulses to about 0.5 mJ/ns that could significantly shorten the usable lifetime of the lens elements.
A pulse stretching configuration has been proposed in U.S. Patent Application Publication 2004/0136417 A1 for use with a lithographic apparatus. In this application, the problem of optical damage is avoided by increasing substantially the pulse length from about 20 ns to more than 50 ns providing a reduction in the rate of optics degradation.
The pulse stretching unit from US Patent Application Publication 2004/0136417 A1 is shown in FIGS. 3 and 16: A 60R/40T beam splitter 10 reflects about 60 percent of the incoming radiation beam 100 into a delay path 40 formed by four focusing mirrors 20A, 20B, 2OC and 20D. The 40 percent transmitted portion of each pulse of beam 100 becomes a first sub-pulse of a corresponding stretched pulse in the outgoing beam 110. The reflected beam is directed by beam splitter 10 to mirror 20A which focuses the reflected portion to point 30. The beam then expands and is reflected from mirror 20B which converts the expanding beam into a parallel beam and directs it to mirror 20C which again focuses the beam again at point 30. This beam is then reflected by mirror 20D which like the 20B mirror changes the expanding beam to a light parallel beam and directs it back to beam splitter 10 where 60 percent of the first reflected light is reflected perfectly in line with the first transmitted portion of this pulse in outgoing beam 110 to become a second sub-pulse. 40 percent of the first reflected beam is transmitted by beam splitter 10 and follows exactly the path of the first reflected beam producing additional smaller sub-pulses in the outgoing radiation beam 110. By configuring the reflection coefficients and the path lengths, the resulting outgoing pulse is stretched from about 20 ns to about 50 ns.
In practice, the known pulse stretching unit, however, has the disadvantage that the lithographic apparatus needs considerable re-calibration after inserting the pulse modifier, which makes upgrading an apparatus in the field time-consuming. Additionally, the reliability will be low because of the need for calibration at regular intervals.